The Fort Worth Star-Telegram reported Friday that Fields, who suffered the injury two weeks ago at Texas Tech, has a broken foot and could be out longer than one game.
Patterson has not commented on the extent of the injury.
Fields, who led the Big 12 last season with 18.5 tackles for loss, has not been a factor so far this year. He didn't play in TCU's opener against LSU and played just one quarter the following week against Southeastern Louisiana while serving out a two-game suspension for breaking unspecified team rules in May.
Fields still has one more quarter to sit out before his suspension is complete.
In his only full game, Fields had just three tackles against Texas Tech, though two of them were for loss in the Horned Frogs' 20-10 loss in Lubbock.
TCU, which received preseason votes to win the Big 12, is off to a 1-2 start.